Soil, sand, aggregate and other particulate material may readily be contaminated with petroleum products and other combustible and/or hazardous materials. Such contamination has been unfortunately all too common at mining and industrial sites and petroleum storage and sales facilities. Consequently, many such sites are contaminated with waste oil, fuel oil and a variety of other combustible and otherwise hazardous materials. Governmental regulations relating to the storage of such materials and to the clean-up of such sites have led to a requirement for the remediation of thousands of tons of soils and other particulate materials from such sites or that are otherwise contaminated with such materials. The two most commonly used methods for remediating or treating contaminated soils and other particulate materials to remove such contaminants are high-temperature incineration and low-temperature thermal desorption.
High-temperature incineration is a process by which the soils or other particulate materials to be treated are heated to a temperature high enough to oxidize or incinerate the contaminants therein. This process is generally capable of treating highly contaminated materials in order to remove the contaminants therefrom; however, it does have several disadvantages. High-temperature incineration requires the consumption of large quantities of fuel and the provision of equipment that is capable of withstanding the high temperatures at which it is carried out. Thus, high-temperature incineration is relatively expensive. Furthermore, it alters the molecular structure of many of the constituents in the material being treated, thus forming a concentrated ash that must be disposed of. In addition, when used to treat soil, high-temperature incineration generally destroys all organic materials therein, not just the undesirable ones, and may render the soil unsuitable for sustaining plant life. This may require that the treated soil be removed from the site to a landfill, instead of being reusable on site. High temperature incineration processes are described in U.S. Pat. No. 4,941,822 of Evans et al. and in U.S. Pat. No. 5,273,355 of May et al.
Low-temperature thermal desorption does not alter the molecular structure of the particulate material treated thereby. Instead, it usually involves a two-step process in which the material is first heated to a temperature sufficient to vaporize the contaminants but lower than that required for incineration thereof. Then, in the second step of the process, the vaporized contaminants are oxidized in a separate high-temperature afterburner. Low-temperature thermal desorption is often preferred to high-temperature incineration for a number of reasons. The first step in the low-temperature thermal desorption process, in which the soil is heated, is carried out at a temperature considerably lower than that required for incineration. In addition, the second step in the low-temperature thermal desorption process, in which the evaporated contaminants are incinerated, requires much less energy than does the incineration of the soil in a high-temperature process. Therefore, the low-temperature thermal desorption process requires much less fuel than does the high-temperature incineration process. In addition, the capital cost for the thermal desorption system is generally less than that of the high-temperature incineration system. The expense in providing equipment that can be operated at the temperatures required for high-temperature incineration is considerable, and because the high-temperature step of the thermal desorption process is carried out in a much smaller incineration chamber than would be required for incineration of all of the soil to be treated, it requires much less of such expensive equipment. In addition, because the high-temperature incineration process frequently renders the treated soil unsuitable for sustaining plant life, the treated soil may have to be trucked offsite to a landfill, thereby greatly increasing the cost of handling it. Low-temperature thermal desorption processes are described in U.S. Pat. No. 5,176,445 of Mize, U.S. Pat. No. 5,382,002 of Evans et al. and U.S. Pat. No. 5,393,501 of Clawson et al.
However, low-temperature thermal desorption processes may also have limitations that restrict their use. Vaporized petroleum and similar contaminants are highly combustible. Therefore, great care must be taken when combustible contaminants are conveyed from a first-stage vaporization chamber to a high-temperature combustion chamber for oxidation. In order to reduce the risk of explosion when such contaminants are removed by vaporization in the first stage of a thermal desorption system, their concentrations must be maintained at levels well below their lower explosive limits. Contaminant concentrations above this limit could cause explosions in the ductwork leading to the afterburner. Thus, thermal desorption systems have generally been considered unsuitable for the treatment of highly contaminated particulate material.
However, modified low-temperature thermal desorption systems have also been developed for the treatment of highly contaminated materials. Thus for example, U.S. Pat. No. 5,176,087 of Noland et al., U.S. Pat. No. 5,230,167 of Lahoda et al., U.S. Pat. No. 5,361,514 of Lahoda et al. and U.S. Pat. No. 5,466,418 of Swanson et al. all describe systems of such type.
All high temperature incineration and low-temperature thermal desorption treatment methods involve the application of heat to the contaminated material, and most such methods require that there be direct contact between the contaminated material and a heat source flame and/or its products of combustion. Where such direct contact occurs, extensive air pollution restrictions generally apply, and permitting for such a direct-contact system may be expensive and time-consuming. Furthermore, the necessary treatment of the contaminated hot gases resulting from such direct contact may be difficult and expensive.
Therefore, it would be desirable if a method and apparatus could be developed for treating contaminated particulate material without exposing the contaminated material to direct contact with the flame and/or products of combustion from a heat source. However, of the conventional and known processes referred to above, only U.S. Pat. No. 4,941,822 of Evans et al. describes a process that does not provide for such direct contact.
The '822 patent of Evans et al. describes an apparatus for incinerating the contaminants in contaminated particulate material, such as foundry sand. According to the Evans process, the sand is fed into an inclined rotating drum which is externally heated to a temperature within the range of 1200.degree.-1600.degree. F. by a plurality of fuel burners mounted within openings in a housing around the drum. Air is supplied to the drum in a quantity in excess of that required to completely burn the organic materials from the sand and calcine the mineral contaminants. However, such method employs an inherently inefficient means of heating the sand in the drum.
It would be desirable therefore, if a more efficient method and apparatus could be developed that could be utilized to treat contaminated materials while avoiding any direct contact between the contaminated material and the flame and/or products of combustion from the heat source. It would also be desirable if a low-temperature thermal desorption method could be developed that could be used to treat a variety of types of contaminated particulate materials, including soils that have been contaminated with petroleum products, without there being any direct contact between the contaminated materials and the flame and/or products of combustion from the heat source.